This invention relates to a display element drive device for driving a single EL display element or a plurality of parallel-connected EL display elements.
FIG. 5 shows the relation between the current density and brightness of an organic EL light emitting element. The coordinates of the current density and brightness are logarithmically represented. Generally, the organic EL light emitting element is a current-driven light emitting element. As shown in FIG. 5, the luminance brightness of the organic EL light emitting element is determined according to the current value per unit area, that is, the current density of the light emitting element. It is, therefore, important for uniforming the brightness and improving display quality to set the current density with good accuracy.
FIG. 6 is a circuit block diagram illustrating an example of an organic EL light emitting element of the dot matrix type. As illustrated in FIG. 6, in the case of using the EL light emitting elements each having a constant area like those of the dot matrix type, while display rows are selected by sink type row drivers 4, all pixels can be driven by a single constant current reference source 1 and a plurality of constant current drivers (that is, source type column drivers) 2a, 2b, 2c, . . . , 2n. Incidentally, in FIG. 6, reference numeral 3 designates each of the organic EL light emitting elements.
Incidentally, in addition to the dot matrix display apparatus, a fixed segment display apparatus has been generally known. Despite the constraint that a display pattern is fixed, this fixed segment display apparatus has advantages in that the display apparatus of this type can display edge portions of curves more beautifully than the display apparatus of the dot matrix type, and that the EL light emitting elements are easily manufactured because of a small number of steps of a manufacturing process thereof. Thus, the fixed segment display apparatus is effectively used in relatively low cost equipment and in field requiring display quality.
Unlike the dot matrix display apparatus, the areas of individual pictures (or segments) differ from one another in the fixed segment display apparatus. Thus, the current values of the driving currents of individual segments differ from one another. Therefore, a plurality of constant current reference sources are needed for causing the segments to emit light with the same brightness.
FIG. 7 is a circuit block diagram illustrating a conventional fixed segment display apparatus. In this apparatus, a plurality of constant current reference sources 5a, 5b, 5c, . . . , 5n supply constant currents to constant current drivers 6a, 6b, 6c, . . . , 6n, respectively. Thus, each of organic EL light emitting segments 7a, 7b, 7c, . . . , 7n is driven.
Thus, in the fixed segment display apparatus, the display pattern varies with the segments. Further, the number of the segments and the areas of the segments vary with apparatuses to which the display pattern is applied. Therefore, it is not preferable from the viewpoint of standardization of the display apparatus to fix a set value of each of the preliminarily prepared constant current sources 5a, 5b, 5c, . . . , 5n for a drive device consisting of the constant current reference sources 5a, 5b, 5c, . . . , 5n and the current drivers 6a, 6b, 6c, . . . , 6n. Consequently, this conventional display apparatuses have a drawback in that a drive device should be custom-designed for each of the display apparatuses. Moreover, the use of the plurality of constant current reference sources 5a, 5b, 5c, . . . , 5n itself hinders the enhancement of the area efficiency of the circuit.
Incidentally, it is possible to use a constant voltage circuit instead of the drive circuit shown in FIG. 7 and parallel-connect all the organic EL light emitting elements with the constant voltage circuit. In this case, the custom-designed constant current reference sources are unnecessary. Consequently, the area efficiency of the circuit can be enhanced.
However, generally, according to the voltage-current characteristic of the organic EL light emitting element, change in the current increases exponentially with increase in the voltage, as illustrated in FIG. 8. Thus, in the case of the drive circuit using the constant voltage circuit, even when a small error occurs in the constant voltage, the current density may largely change. Consequently, there is a fear that the brightness of the organic EL light emitting element largely changes, and the display quality is deteriorated. It is, therefore, necessary to precisely adjust the voltage supply. Consequently, the provision of a more complex voltage stabilization circuit is needed. Especially, in the case that organic EL light emitting elements in an automobile instrument panel are driven by being supplied with power from an automobile battery, there is the necessity for applying voltages to drive loads other than a power steering device and a power window device. Thus, there has been a problem of how to achieve the stabilization of a supply voltage.
Additionally, the resistance value of the organic EL light emitting element may change owing to the deterioration thereof and to the influence of the ambient temperature, so that the driving current changes. Consequently, there has been a problem of how to stabilize the brightness of the organic EL light emitting element.
In view of the problems of the conventional example, in the Japanese Patent Application No. 10-301188, the Applicants of the present application have proposed a display element drive device (namely, a proposed device example), which serves as a display element drive circuit enabled to increase the area efficiency of the circuit, and to be adapted to standardization, and to cause small change in the luminance brightness of display elements when the display elements are supplied with power from an automobile battery that is relatively liable to bring about voltage variation, and to stably maintain the luminance brightness even when the resistance value of the display element changes owing to the deterioration thereof, and to have excellent durability.
In this proposed device example, as illustrated in FIGS. 9 and 10, a plurality of fixed segment organic EL display elements 11a to 11n are parallel-connected to one another, and a stabilization voltage is supplied to the parallel circuit. Thus, the plurality of conventional drive reference sources (namely, the current sources) needed owing to the difference in the area among the segments are omitted. Moreover, the segments are allowed to have the same brightness.
Further, to deal with variation in characteristics and aged deterioration in the voltage-driven case, the device has a current detecting means 31 (a drive state detecting means) for detecting the current value of electric current supplied to one specific organic EL display element (hereunder referred to as xe2x80x9creference organic EL display elementsxe2x80x9d) 11z (reference light emitting element) other than the organic EL display elements 11a to 11n, and for outputting a current value signal adapted to change according to the electric current value, a voltage control circuit 32 for converting a current value signal, which is received from the current detecting means 31, into a stabilization voltage adjustment signal, and a stabilization voltage supply circuit 33 for converting a voltage Vin, which is supplied from an astable battery power supply (+B), into a constant stabilization voltage Vout.
Incidentally, the reference organic EL display element 11z is connected to the current detecting means 31, and supplied with electric current from the current detecting means 31. On the other hand, other organic EL display elements 11a, . . . , 11n are supplied with electric current through predetermined switching circuits 15 (15a to 15n), as illustrated in FIG. 10.
Each of the switching circuits 15 (15a to 15n) has a PNP transistor Q6 for supplying driving currents Ia to In to the organic EL display elements 11a to 11n, and an NPN transistor Q5 for switching on and off the transistor PNP. The base of the PNP transistor Q6 is connected to the collector of the NPN transistor Q5 through a resistor R5. Moreover, the base of the NPN transistor Q5 is connected to the control portion 13 through a resistor R4. Furthermore, the emitter of the NPN transistor Q5 is grounded. These switching circuits 15 (15a to 15n) are parallel-connected to one another. Further, a common stabilization voltage Vout is applied to the switching circuits 15 (15a to 15n). Incidentally, as illustrated in FIG. 9, each of the switching circuits 15 (15a to 15n) is switched on and off according to a switching signal outputted from the control portion 13.
The current detecting means 31 is used for detecting the driving state of a single reference organic EL display element 11z by sensing the current value of electric current supplied to the reference organic EL display element. Further, the current detecting means 13 has a single current detector Rref, a single operational amplifier A-2, and four resistors Rf1, Rf2, Rf3 and Rs interposed between the reference organic EL display element 11z and the stabilization voltage supply circuit 33.
The inverting input terminal of the operational amplifier A-2 is connected to the resistor so that an output of the operational amplifier A-2 is negative-fed back thereto. The inverting input terminal is also connected to the connecting point between the resistor Rf1 and the reference organic EL display element 11z. Further, the noninverting input terminal of the operational amplifier A-2 is connected to the connecting point between the current detecting resistor Rref and the stabilization voltage supply circuit 33 through the resistor Rs, and grounded through the resistor Rf3. With such a circuit configuration, the operational amplifier A-2 functions as a differential amplifier for converting a voltage developed across the current detecting resistor Rref into a current value signal V.
Incidentally, a pair of resistors Rf3 and Rs connected to the noninverting input terminal of the operational amplifier A-2 serves as voltage divider resistors for generating a partial voltage of the stabilization voltage Vout (Rrefxc3x97Iref) Let Iref designate electric current flowing through the current detecting resistor Rref. Moreover, let xcex1 denote a dividing ratio (=Rf3/Rs), at which the stabilization voltage is divided by using the voltage dividing resistors Rf3 and Rs. Furthermore, in the case that Rf2=Rs (namely, xcex1=Rf3/Rf2), and that Rf1=Rf3, the current value V represented by the current value signal is expressed by the equation (1):
V32 (Rf3/Rf2)xc3x97Rrefxc3x97Iref=xcex1xc3x97Rrefxc3x97Irefxe2x80x83xe2x80x83(1)
Further, the resistance value of the current detecting resistor Rref is set in such a manner as to be sufficiently small value in comparison with the resistance values of the segments Rz, Ra, Rb, . . . , Rn (Rm) of the segments (namely, the organic EL display elements 11z, 11a to 11n). Moreover, Irefxc2x7Rref is set in such a way as to be nearly equal to the forward voltage of the PNP transistor Q6 turned on and provided in each of the switching circuits 15 (15a to 15n), voltages respectively applied to the reference organic EL display element 11z and other segments (namely, the organic EL display elements 11a, . . . , 11n) can be made to be almost equal to one another.
The voltage control circuit 32 consists of a single operational amplifier A-1, a single resistor R1, and a Zener diode ZD1 serving as a single constant voltage element. The noninverting input terminal of the operational amplifier A-1 is connected to the cathode of the Zener diode ZD1 and grounded through the Zener diode ZD1. Further, the inverting input terminal of the operational amplifier A-1 is connected to the current detecting means 31. Furthermore, the operational amplifier A-1 is adapted to control an output thereof so that the voltage V (=xcex1xc3x97Rrefxc3x97Iref) applied from the current detecting means 31 to the noninverting input terminal thereof is made to be approximately equal to the backward voltage Vz provided thereto by being connected to the Zener diode ZD1. Further, the cathode of the Zener diode ZD1 is connected to the battery power supply (+B) through the resistor R11. Incidentally, the operational amplifier A-1 is adapted to ensure a positive power value of an output thereof, which value is sufficient to the extent that the transistor Q11 can output the voltage Vout at all times.
The stabilization voltage supply circuit 33 is practically constituted by a single NPN transistor Q11. Further, the circuit 33 converts the voltage Vin, which is supplied from the battery power supply (+B), to the stabilization voltage Vout serving as emitter potential, according to base potential provided from the voltage control circuit 32 to the circuit 33. Then, the circuit 33 outputs the voltage Vout to the switching circuits 15 (15a to 15n) and the current detecting resistor Rref of the current detecting means 31.
In the case of the proposed device example of the aforementioned configuration, first, when the voltage Vin is supplied from the battery power supply (+B) through the resistor R11 to the Zener diode ZD1 serving as a constant voltage element, the voltage at the noninverting input terminal of the operational amplifier A-1 of the voltage control circuit 32 is fixed by the Zener diode ZD1 at a constant voltage Vz. The operational amplifier A-1 outputs a stabilization adjustment signal, which is used for equalizing the voltage V to the constant voltage Vz, according to the voltage V(=xcex1xc2x7Rrefxc2x7Iref) supplied to the inverting input terminal thereof from the current detecting means 31 and to the constant voltage Vz.
The stabilization voltage supply circuit 33 (Q11) converts the instable power supply voltage Vin to the constant stable voltage Vout in response to a stabilization voltage adjustment signal outputted from the voltage control circuit 32.
At that time, in the case that the stabilization voltage Vout is supplied to the parallel connecting points in the organic EL display elements 11z, and 11a to 11n, the application voltage (namely, the stabilization voltage Vout) is equally applied to all the segments (namely, the organic EL display elements 11z, 11a to 11n). Thus, electric currents Im (Iref, Ia, Ib, . . . , In) each having a current value, which is in inverse proportion to the resistance values Rm of the segments, flow therethrough. In this way, the currents are automatically adjusted so that the current density becomes constant correspondingly to the area of each of the segments. Consequently, the luminance brightnesses of all the segments (namely, the organic EL display elements 11z, 11a to 11n) are stabilized without being affected by variation in the power supply voltage Vin.
Meanwhile, generally, the resistance values of the organic EL display elements 11a to 11n, 11z are changed owing to the deterioration of the display elements, which is caused over years of use, and to the change in the ambient temperature. However, in the case that the voltage to be applied to the organic EL display elements 11a to 11n, 11z is maintained at a fixed value, the current values of currents flowing through the organic EL display elements 11a to 11n, 11z change. This results in variation in the luminance brightness.
However, even in such a case, in this proposed device example, the luminance brightness of each of the organic EL display elements 11a to 11n, 11z is stably maintained by adjusting the voltage Vout according to the change in the resistance value of each of the elements 11a to 11n, 11z. 
That is, the operational amplifier A-2 of the current detecting means 31 functions as a differential amplifier for converting a voltage developed across the current detecting resistor Rref into a current value signal V. Further, the operational amplifier A-2 outputs the current value signal V according to the equation (1) to the voltage control circuit 32. Moreover, as described above, a stabilization adjustment signal for equalizing the voltage V (=xcex1xc2x7Rrefxc2x7Iref), which is supplied from the current detecting means 31 to the inverting input terminal, to the constant voltage value Vz is outputted by the operational amplifier A-1 of the voltage control circuit 32. The stabilization voltage supply circuit 33 (All) adjusts the voltage value of the stabilization voltage Vout according to an output of the voltage control circuit 32. That is, when the resistance (Rz) of the reference organic EL display element 11z lowers owing to the deterioration of the elements, the voltage Vout lowers. Conversely, when the resistance (Rz) rises, the voltage Vout rises. Needless to say, after the change in the resistance (Rz), the voltage control is performed so that the voltage V is stably maintained at a value of Vout even when the power supply voltage Vin changes.
Thus, even when the resistance (Rz) of the reference organic EL display element 11z changes owing to the deterioration thereof, the voltage Vout applied to each of the switching circuits 15 (15a to 15n) and the current detecting resistance Rref is adjusted, so that electric current supplied to each of the organic EL display elements 11a to 11n is stabilized thereby to maintain the brightness thereof at a constant value.
Incidentally, in the case that all the organic EL display elements 11z, 11a to 11n have nearly the same voltage-current characteristics and changes thereof with time, even when the internal resistances (Rz, Ra, Rb, . . . , Rn) of the segments change owing to the variation in the characteristics, the voltage Vout is controlled so that each of the driving currents Iref, Ia, Ib, . . . , In has a constant current value. Thus, change in the brightness of each of the display elements is small, in comparison with that in the case of employing a simple constant voltage driving method. Further, the difference in brightness among the segments is decreased.
Generally, when the organic EL display elements deteriorate with time, leakage current may abruptly increase in a part of segments. Thus, at an occurrence of an abnormal condition, such as an abrupt increase in leakage current, display segments, in each of which the abnormal condition occurs, stop emitting light. Moreover, an amount of heat generated by the wiring resistance of transparent electrodes of each of the organic EL display elements is increased. The generated heat adversely affects not only such abnormal display segments but also other normal display segments. Thus, the deterioration of the organic light emitting layers of surrounding display segments is promoted. Furthermore, the leakage current becomes an overcurrent, so that various kinds of drive circuits for driving display segments are destroyed. Thus, such destruction of the drive circuits may bring the entire EL display apparatus into an inoperative condition. The proposed device example cannot prevent an occurrence of such an inoperative condition thereof. Consequently, there is the necessity for improving the proposed device example.
Accordingly, the problem to be solved by the present invention is to provide a display element drive device adapted to impose certain limits on driving currents, which are supplied to display segments, even when the display segments are partly deteriorated and leakage current increases, thereby to contribute to the prevention of heat generation and destruction of various kinds of drive circuits.
To solve the foregoing problems, according to a first aspect of the present invention, there is provided a display element drive device for driving a single EL display element or a plurality of EL display elements parallel-connected to one another, which comprises a single stabilization voltage supply circuit for applying a stabilization voltage to the EL display element, a reference EL display element parallel-connected to the EL display element, driving state detecting means for detecting a driving state and for changing an output signal according to the driving state, a voltage control circuit for controlling a constant voltage by supplying a stabilization voltage adjustment signal to the stabilization voltage supply circuit according to the output signal of the driving state detecting means so that the driving state of the reference EL display element is constant, and a switching circuit for switching between application and disapplication of the stabilization voltage to the EL display element. In the device, at least one of the stabilization voltage supply circuit and the switching circuit has a transistor for supplying a driving current to the EL display element. The transistor is connected to a current detecting element for detecting a collector current of the transistor, and to a bias suppressing element for reducing a base-emitter bias of the transistor by on-switching when the current detected by the current detecting element rises to a certain abnormal level owing to leakage current of the EL display element.
According to a second aspect of the present invention, there is provided a display element drive device for driving a single EL display element or a plurality of EL display elements parallel-connected to one another, which comprises a single stabilization voltage supply circuit for applying a stabilization voltage to the EL display element, a reference EL display element parallel-connected to the EL display element, a control portion for detecting a driving state and for changing a control signal, which is used to control the stabilization voltage, according to the driving state, a voltage control circuit for controlling a constant voltage by supplying a stabilization voltage adjustment signal to the stabilization voltage supply circuit according to the control signal outputted from the control portion so that the driving state of the reference EL display element is constant, and a switching circuit for switching between application and disapplication of the stabilization voltage to the EL display element. In the device, the switching circuit is adapted to perform on-off switching according to a switching signal sent from the control portion. The stabilization voltage supply circuit has a stabilization voltage supply element for adjusting an output level of the stabilization voltage according to the stabilization voltage adjustment signal supplied from the voltage control circuit, and further has a current detecting element for detecting a current outputted from the stabilization voltage supply element. The control portion has a function of outputting to the switching circuit a switching signal for applying the stabilization voltage to the EL display element by on-switching of the switching circuit. The control portion further has a function of judging that leakage current of the EL display element abnormally increases, and changing the control signal to thereby limit the output level of the stabilization voltage outputted from the stabilization voltage supply element when the current detected by the current detecting element rises to an abnormally high level in comparison with a level of a driving current needed for driving the EL display element to emit light in a case that the on-switching of the switching circuit is performed in response to the switching signal.